Ethelred succeeded to the throne aged 10 following the death of his father King Edgar and subsequent murder of his half-brother Edward the Martyr. His nickname "The Unready" does not mean that he was ill-prepared, but derives from the Anglo-Saxonunrĉd meaning without counsel. This is also a pun on his name, the Anglo-Saxon form of his name, Ĉŝelred, which means "Well advised".

Ethelred had at least sixteen children from two marriages, the second of these, in 1002, being to Emma of Normandy, whose great-nephew, William I of England, would later use this relationship as the basis of his claim on the throne.

He attempted to buy off the Vikings by payment of what was to become known as Danegeld; he had little choice in the matter since he was unable to place any trust in his generals.